Internet Engineering Task Force (IETF) K. Drage
Request for Comments: 6050 Alcatel-Lucent
Category: Informational November 2010
ISSN: 2070-1721
A Session Initiation Protocol (SIP) Extensionfor the Identification of Services
Abstract
This document describes private extensions to the Session Initiation
Protocol (SIP) that enable a network of trusted SIP servers to assert
the service of authenticated users. The use of these extensions is
only applicable inside an administrative domain with previously
agreed-upon policies for generation, transport, and usage of such
information. This document does NOT offer a general service
identification model suitable for use between different trust domains
or for use in the Internet at large.
The document also defines a URN to identify both services and User
Agent (UA) applications. This URN can be used within the SIP header
fields defined in this document to identify services, and also within
the framework defined for caller preferences and callee capabilities
to identify usage of both services and applications between end UAs.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6050.
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RFC 6050 SIP Service Identification November 20101. Introduction
This document describes private extensions to the Session Initiation
Protocol (SIP) that enable a network of trusted SIP servers to assert
the service, possibly subject to the user being entitled to that
service. The use of these extensions is only applicable inside an
administrative domain with previously agreed-upon policies for
generation, transport, and usage of such information. This document
does NOT offer a general service model suitable for use between
different trust domains or for use in the Internet at large.
The concept of "service" within SIP has no hard and fast rules. RFC5897 [RFC5897] provides general guidance on what constitutes a
service within SIP and what does not.
This document also makes use of the terms "derived service
identification" and "declarative service identification" as defined
in RFC 5897 [RFC5897].
It should be noted that RFC 5897 [RFC5897] clearly states that
declarative service identification -- the process by which a user
agent inserts a moniker into a message that defines the desired
service, separate from explicit and well-defined protocol mechanisms
-- is harmful.
During a session setup, proxies may need to understand what service
the request is related to in order to know what application server to
contact or other service logic to invoke. The SIP INVITE request
contains all of the information necessary to determine the service.
However, the calculation of the service may be computational and
database intensive. For example, a given trust domain's definition
of a service might include request authorization. Moreover, the
analysis may require examination of the Session Description Protocol
(SDP).
For example, an INVITE request with video SDP directed to a video-on-
demand Request-URI could be marked as an IPTV session. An INVITE
request with push-to-talk over cellular (PoC) routes could be marked
as a PoC session. An INVITE request with a Require header field
containing an option tag of "foogame" could be marked as a foogame
session.
NOTE: If the information contained within the SIP INVITE request is
not sufficient to uniquely identify a service, the remedy is to
extend the SIP signaling to capture the missing element. RFC 5897
[RFC5897] provides further explanation.
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RFC 6050 SIP Service Identification November 2010
By providing a mechanism to compute and store the results of the
domain-specific service calculation, i.e., the derived service
identification, this optimization allows a single trusted proxy to
perform an analysis of the request and authorize the requestor's
permission to request such a service. The proxy may then include a
service identifier that relieves other trusted proxies and trusted
UAs from performing further duplicate analysis of the request for
their service identification purposes. In addition, this extension
allows user agent clients outside the trust domain to provide a hint
of the requested service.
This extension does not provide for the dialog or transaction to be
rejected if the service is not supported end-to-end. SIP provides
other mechanisms, such as the option-tag and use of the Require and
Proxy-Require header fields, where such functionality is required.
No explicitly signaled service identification exists, and the session
proceeds for each node's definition of the service in use, on the
basis of information contained in the SDP and in other SIP header
fields.
This mechanism is specifically for managing the information needs of
intermediate routing devices between the calling user and the user
represented by the Request-URI. In support of this mechanism, a URN
is defined to identify the services. This URN has wider
applicability to additionally identify services and terminal
applications. Between end users, caller preferences and callee
capabilities as specified in RFC 3840 [RFC3840] and RFC 3841
[RFC3841] provide an appropriate mechanism for indicating such
service and application identification. These mechanisms have been
extended by RFC 5688 [RFC5688] to provide further capabilities in
this area.
The mechanism proposed in this document relies on a new header field
called 'P-Asserted-Service' that contains a URN. This is supported
by a further new header field called 'P-Preferred-Service' that also
contains a URN and that allows the UA to express preferences
regarding the decisions made on service within the trust domain.
An example of the P-Asserted-Service header field is:
P-Asserted-Service: urn:urn-7:3gpp-service.exampletelephony.version1
A proxy server that handles a request can, after authenticating the
originating user in some way (for example: digest authentication) to
ensure that the user is entitled to that service, insert such a
P-Asserted-Service header field into the request and forward it to
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other trusted proxies. A proxy that is about to forward a request to
a proxy server or UA that it does not trust removes all the
P-Asserted-Service header field values.
This document labels services by means of an informal URN. This
provides a hierarchical structure for defining services and
subservices, and provides an address that can be resolvable for
various purposes outside the scope of this document, e.g., to obtain
information about the service so described.
2. Applicability Statement
This document describes private extensions to SIP (see RFC 3261
[RFC3261]) that enable a network of trusted SIP servers to assert the
service of end users or end systems. The use of these extensions is
only applicable inside a 'trust domain' as defined in "Short Term
Requirements for Network Asserted Identity" (see RFC 3324 [RFC3324]).
Nodes in such a trust domain are explicitly trusted by its users and
end systems to publicly assert the service of each party, and that
they have common and agreed-upon definitions of services and
homogeneous service offerings. The means by which the network
determines the service to assert is outside the scope of this
document (though it commonly entails some form of authentication).
The mechanism for defining a trust domain is to provide a certain set
of specifications known as 'Spec(T)', and then specify compliance to
that set of specifications. Spec(T) MUST specify behavior as
documented in RFC 3324 [RFC3324].
This document does NOT offer a general service model suitable for
inter-domain use or use in the Internet at large. Its assumptions
about the trust relationship between the user and the network may not
apply in many applications. For example, these extensions do not
accommodate a model whereby end users can independently assert their
service by use of the extensions defined here. End users assert
their service by including the SIP and SDP parameters that correspond
to the service they require. Furthermore, since the asserted
services are not cryptographically certified, they are subject to
forgery, replay, and falsification in any architecture that does not
meet the requirements of RFC 3324 [RFC3324].
The asserted services also lack an indication of who specifically is
asserting the service, and so it must be assumed that a member of the
trust domain is asserting the service. Therefore, the information is
only meaningful when securely received from a node known to be a
member of the trust domain.
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Despite these limitations, there are sufficiently useful specialized
deployments, that meet the assumptions described above and can accept
the limitations that result, to warrant informational publication of
this mechanism.
3. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
Throughout this document, requirements for or references to proxy
servers or proxy behavior apply similarly to other intermediaries
within a trust domain (for example, back-to-back user agents
(B2BUAs)).
The term trust domain in this document has the meaning as defined in
RFC 3324 [RFC3324].
4. Syntax of the Header Fields
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC 5234 [RFC5234].
4.1. The P-Asserted-Service Header
The P-Asserted-Service header field is used among trusted SIP
entities (typically intermediaries) to carry the service information
of the user sending a SIP message.
The P-Asserted-Service header field carries information that is
derived service identification. While a declarative service
identification can assist in deriving the value transferred in this
header field, this should be in the form of streamlining the correct
derived service identification.
PAssertedService = "P-Asserted-Service"
HCOLON PAssertedService-value
PAssertedService-value = Service-ID *(COMMA Service-ID)
See Section 4.4 for the definition of Service-ID in ABNF.
Proxies can (and will) add and remove this header field.
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RFC 6050 SIP Service Identification November 2010
Table 1 adds the header fields defined in this document to Table 2 in
SIP [RFC3261], Section 7.1 of the SIP-specific event notification
[RFC3265], Tables 1 and 2 in the SIP INFO method [RFC2976], Tables 1
and 2 in the reliability of provisional responses in SIP [RFC3262],
Tables 1 and 2 in the SIP UPDATE method [RFC3311], Tables 1 and 2 in
the SIP extension for instant messaging [RFC3428], Table 1 in the SIP
REFER method [RFC3515], and Tables 2 and 3 in the SIP PUBLISH method
[RFC3903]:
Header field where proxy ACK BYE CAN INV OPT REG SUB
_______________________________________________________________
P-Asserted-Service R admr - - - o o - o
Header field NOT PRA INF UPD MSG REF PUB
_______________________________________________________________
P-Asserted-Service - - - - o o o
Table 1
Syntactically, there may be multiple P-Asserted-Service header fields
in a request. The semantics of multiple P-Asserted-Service header
fields appearing in the same request is not defined at this time.
Implementations of this specification MUST provide only one
P-Asserted-Service header field value.
4.2. The P-Preferred-Service Header
The P-Preferred-Service header field is used by a user agent sending
the SIP request to provide a hint to a trusted proxy of the preferred
service that the user wishes to be used for the P-Asserted-Service
field value that the trusted element will insert.
The P-Preferred-Service header field carries information that is
declarative service identification. Such information should only be
used to assist in deriving a derived service identification at the
recipient entity.
PPreferredService = "P-Preferred-Service"
HCOLON PPreferredService-value
PPreferredService-value = Service-ID *(COMMA Service-ID)
See Section 4.4 for the definition of Service-ID in ABNF.
Table 2 adds the header fields defined in this document to Table 2 in
SIP [RFC3261], Section 7.1 of the SIP-specific event notification
[RFC3265], Tables 1 and 2 in the SIP INFO method [RFC2976], Tables 1
and 2 in Reliability of provisional responses in SIP [RFC3262],
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Tables 1 and 2 in the SIP UPDATE method [RFC3311], Tables 1 and 2 in
the SIP extension for Instant Messaging [RFC3428], Table 1 in the SIP
REFER method [RFC3515], and Tables 2 and 3 in the SIP PUBLISH method
[RFC3903]:
Header field where proxy ACK BYE CAN INV OPT REG SUB
_______________________________________________________________
P-Preferred-Service R dr - - - o o - o
Header field NOT PRA INF UPD MSG REF PUB
_______________________________________________________________
P-Preferred-Service - - - - o o o
Table 2
Syntactically, there may be multiple P-Preferred-Service header
fields in a request. The semantics of multiple P-Preferred-Service
header fields appearing in the same request is not defined at this
time. Implementations of this specification MUST only provide one
P-Preferred-Service header field value.
4.3. Service and Application Definition
Service definitions and characteristics are outside the scope of this
document. Other standards organizations, vendors, and operators may
define their own services and register them.
A hierarchical structure is defined consisting of service identifiers
or application identifiers, and subservice identifiers.
The service and subservice identifiers are as described in Section 1.
The URN may also be used to identify a service or an application
between end users for use within the context of RFC 3840 [RFC3840]
and RFC 3841 [RFC3841].
IANA maintains a registry of service identifier values that have been
assigned. This registry has been created by the actions of Section8.2 of this document.
subservice identifiers are not managed by IANA. It is the
responsibility of the organization that registered the service to
manage the subservices.
4.4. Registration Template
Below, we include the registration template for the URN scheme
according to RFC 3406 [RFC3406]. The URN scheme is defined as an
informal Namespace ID (NID).
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Namespace ID: urn-7
Registration Information:
Registration version: 1; registration date: 2009-03-22
Declared registrant of the namespace: 3GPP Specifications Manager
(3gppContact@etsi.org) (+33 (0)492944200)
Declaration of syntactic structure: The URN consists of a
hierarchical service identifier or application identifier, with a
sequence of labels separated by periods. The leftmost label is
the most significant one and is called 'top-level service
identifier', while names to the right are called 'subservices' or
'sub-applications'. The set of allowable characters is the same
as that for domain names (see RFC 1123 [RFC1123]) and a subset of
the labels allowed in RFC 3958 [RFC3958]. Labels are case-
insensitive and MUST be specified in all lowercase. For any given
service identifier, labels can be removed right-to-left and the
resulting URN is still valid, referring a more generic service,
with the exception of the top-level service identifier and
possibly the first subservice or sub-application identifier.
Labels cannot be removed beyond a defined basic service; for
example, the label w.x may define a service, but the label w may
only define an assignment authority for assigning subsequent
values and not define a service in its own right. In other words,
if a service identifier 'w.x.y.z' exists, the URNs 'w.x' and
'w.x.y' are also valid service identifiers, but w may not be a
valid service identifier if it merely defines who is responsible
for defining x.
Service-ID = "urn:urn-7:" urn-service-id
urn-service-id = top-level *("." sub-service-id)
top-level = let-dig [ *26let-dig ]
sub-service-id = let-dig [ *let-dig ]
let-dig = ALPHA / DIGIT / "-"
While the naming convention above uses the term "service", all the
constructs are equally applicable to identifying applications
within the UA.
Relevant ancillary documentation: None
Identifier uniqueness considerations: A service identifier
identifies a service, and an application identifier an application
indicated in the service or application registration (see IANA
Considerations (Section 8)). Uniqueness is guaranteed by the IANA
registration.
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Identifier persistence considerations: The service or application
identifier for the same service or application is expected to be
persistent, although there naturally cannot be a guarantee that a
particular service will continue to be available globally or at
all times.
Process of identifier assignment: The process of identifier
assignment is described in the IANA Considerations (Section 8).
Process for identifier resolution: There is no single global
resolution service for service identifiers or application
identifiers.
Rules for lexical equivalence: 'service' identifiers are compared
according to case-insensitive string equality.
Conformance with URN syntax: The BNF in the 'Declaration of
syntactic structure' above constrains the syntax for this URN
scheme.
Validation mechanism: Validation determines whether a given string
is currently a validly assigned URN (see RFC 3406 [RFC3406]). Due
to the distributed nature of usage and since not all services are
available everywhere, validation in this sense is not possible.
Scope: The scope for this URN can be local to a single domain, or
may be more widely used.
5. Usage of the P-Preferred-Service and P-Asserted-Service Header Fields5.1. Usage of the P-Preferred-Service and P-Asserted-Service Header Fields in Requests5.1.1. Procedures at User Agent Clients (UAC)
The UAC MAY insert a P-Preferred-Service in a request that creates a
dialog, or a request outside of a dialog. This information can
assist the proxies in identifying appropriate service capabilities to
apply to the call. This information MUST NOT conflict with other SIP
or SDP information included in the request. Furthermore, the SIP or
SDP information needed to signal functionality of this service MUST
be present. Thus, if a service requires a video component, then the
SDP has to include the media line associated with that video
component; it cannot be assumed from the P-Preferred-Service header
field value. Similarly, if the service requires particular SIP
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RFC 6050 SIP Service Identification November 2010
functionality for which a SIP extension and a Require header field
value is defined, then the request has to include that SIP signaling
as well as the P-Preferred-Service header field value.
A UAC that is within the same trust domain as the proxy to which it
sends a request (e.g., a media gateway or application server) MAY
insert a P-Asserted-Service header field in a request that creates a
dialog, or a request outside of a dialog. This information MUST NOT
conflict with other SIP or SDP information included in the request.
Furthermore, the SIP or SDP information needed to signal
functionality of this service MUST be present.
5.1.2. Procedures at Intermediate Proxies
A proxy in a trust domain can receive a request from a node that it
trusts or a node that it does not trust. When a proxy receives a
request from a node it does not trust and it wishes to add a
P-Asserted-Service header field, the proxy MUST identify the service
appropriate to the capabilities (e.g., SDP) in the request, MAY
authenticate the originator of the request (in order to determine
whether the user is subscribed for that service). Where the
originator of the request is authenticated, the proxy MUST use the
identity that results from this checking and authentication to insert
a P-Asserted-Service header field into the request.
When a proxy receives a request containing a P-Preferred-Service
header field, the Proxy MAY use the contents of that header field to
assist in determining the service to be included in a P-Asserted-
Service header field (for instance, to prioritize the order of
comparison of filter criteria for potential services that the request
could match). The proxy MUST NOT use the contents of the
P-Preferred-Service header field to identify the service without
first checking against the capabilities (e.g., SDP) contained in the
request. If the proxy inserts a P-Asserted-Service header field in
the request, the proxy MUST remove the P-Preferred-Service header
field before forwarding the request; otherwise, the Proxy SHOULD
include the P-Preferred-Service header field when forwarding the
request.
If the proxy receives a request from a node that it trusts, it can
use the information in the P-Asserted-Service header field, if any,
as if it had authenticated the user itself.
If there is no P-Asserted-Service header field present, or it is not
possible to match the request to a specific service as identified by
the service identifier, a proxy MAY add one containing it using its
own analysis of the information contained in the SIP request. If the
proxy received the request from an element that it does not trust and
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RFC 6050 SIP Service Identification November 2010
there is a P-Asserted-Service header present, the proxy MUST replace
that header field's contents with a new analysis or remove that
header field.
The analysis performed to identify such service identifiers is
outside the scope of this document. However, it is perfectly valid
as a result of the analysis not to include any service identifier in
the forwarded request, and thus not include a P-Asserted-Service
header field.
If a proxy forwards a request to a node outside the proxy's trust
domain, there MUST NOT be a P-Asserted-Service header field in the
forwarded request.
5.1.3. Procedures at User Agent Servers
For a User Agent Server (UAS) outside the trust domain, the
P-Asserted-Service header is removed before it reaches this entity;
therefore, there are no procedures for such a device.
However, if a UAS receives a request from a previous element that it
does not trust, it MUST NOT use the P-Asserted-Service header field
in any way.
If a UA is part of the trust domain from which it received a request
containing a P-Asserted-Service header field, then it can use the
value freely, but it MUST ensure that it does not forward the
information to any element that is not part of the trust domain.
5.2. Usage of the P-Preferred-Service and P-Asserted-Service Header Fields in Responses
There is no usage of these header fields in responses.
6. Examples of Usage
In this example, proxy.example.com creates a P-Asserted-Service
header field from the user identity it discovered from SIP digest
authentication, the list of services appropriate to that user, and
the services that correspond to the SDP information included in the
request. Note that F1 and F2 are about identifying the user and do
not directly form part of the capability provided in this document.
It forwards this information to a trusted proxy that forwards it to a
trusted gateway. Note that these examples consist of partial SIP
messages that illustrate only those header fields relevant to the
authenticated identity problem.
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* F5 proxy.pstn.example -> gw.pstn.example (trusted)
INVITE sip:+14085551212@gw.pstn.example SIP/2.0
Via: SIP/2.0/TCP useragent.example.com;branch=z9hG4bK-124
Via: SIP/2.0/TCP proxy.example.com;branch=z9hG4bK-abc
Via: SIP/2.0/TCP proxy.pstn.example;branch=z9hG4bK-a1b2
To: <sip:+14085551212@example.com>
From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=9802748
Call-ID: 245780247857024504
CSeq: 2 INVITE
Max-Forwards: 68
P-Asserted-Service: urn:urn-7:3gpp-service.exampletelephony.version1
v=0
o=- 2987933615 2987933615 IN IP6 5555::aaa:bbb:ccc:ddd
s=-
c=IN IP6 5555::aaa:bbb:ccc:ddd
t=0 0
m=audio 3456 RTP/AVPF 97 96
b=AS:25.4
a=curr:qos local sendrecv
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=sendrecv
a=rtpmap:97 AMR
a=fmtp:97 mode-set=0,2,5,7; maxframes
7. Security Considerations
The mechanism provided in this document is a partial consideration of
the problem of service identification in SIP. For example, these
mechanisms provide no means by which end users can securely share
service information end-to-end without a trusted service provider.
This information is secured by transitive trust, which is only as
reliable as the weakest link in the chain of trust.
The trust domain provides a set of servers where the characteristics
of the service are agreed for that service identifier value, and
where the calling user is entitled to use that service. RFC 5897
[RFC5897] identifies the impact of allowing such service identifier
values to "leak" outside of the trust domain, including implications
on fraud, interoperability, and stifling of service innovation.
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RFC 6050 SIP Service Identification November 20108. IANA Considerations8.1. P-Asserted-Service and P-Preferred-Service Header Fields
This document specifies two new SIP header fields: P-Asserted-Service
and P-Preferred-Service. Their syntax is given in Section 3. These
header fields are defined by the following information, which has
been added to the header sub-registry under http://www.iana.org.
Header Name compact Reference
----------------- ------- ---------
P-Asserted-Service RFC 6050
P-Preferred-Service RFC 60508.2. Definition of Service-ID Values
Top-level identifiers are identified by labels managed by IANA,
according to the processes outlined in RFC 5226 [RFC5226], in a new
registry called "Service-ID/Application-ID Labels". Thus, creating a
new service at the top-level requires IANA action. The policy for
adding service labels is 'specification required'. The following two
identifiers are initially defined:
3gpp-service
3gpp-application
subservice identifiers are not managed by IANA. It is the
responsibility of the organization that registered the service to
manage the subservices.
Application identifiers are not managed by IANA. It is the
responsibility of the organization that registered the service to
manage the applicable applications.
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